A long-recognized important objective in the constant advancement of monolithic IC (Integrated Circuit) technology is the scaling-down of IC dimensions. Such scaling-down of IC dimensions reduces area capacitance and is critical to obtaining higher speed performance of integrated circuits. Moreover, reducing the area of an IC die leads to higher yield in IC fabrication. Such advantages are a driving force to constantly scale down IC dimensions.
Referring to FIG. 1, a common component of a monolithic IC is a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) 100 which is fabricated within a semiconductor substrate 102. The conventional MOSFET 100 includes a drain 104, a source 106, and a gate structure 108 disposed over a gate dielectric 110, as known to one of ordinary skill in the art of integrated circuits.
A limitation to further scaling down the dimensions of the MOSFET 100 is scaling down the thickness of the gate dielectric 110. When the thickness of the gate dielectric 110 is scaled down to below approximately 15 .ANG. (angstroms), charge carriers tunnel through such a thin gate dielectric 110 to form undesired tunneling current through the gate 108. With such tunneling current, the gate 108 of the MOSFET 100 becomes less capacitive and more resistive, and the MOSFET 100 has lower current drive capability because less charge is induced in the channel region of the MOSFET 100 below the gate dielectric 110. Such lowered current drive capability degrades the speed performance of the MOSFET 100.
Referring to FIG. 2, another common component of a monolithic IC is a BJT (Bipolar Junction Transistor) 200. The conventional BJT 200 includes an emitter region 122 and a collector region 124 that are comprised of a semiconductor material having a first type of dopant, as known to one of ordinary skill in the art of integrated circuits. The conventional BJT also includes a base region 126 that is comprised of a semiconductor material having a second type of dopant that is opposite of the first type of dopant, as known to one of ordinary skill in the art of integrated circuits.
For example, for an NPN BJT, the emitter region 122 and the collector region 124 may be comprised of silicon with an N-type dopant while the base region 126 may be comprised of silicon with a P-type dopant. Alternatively, for a PNP BJT, the emitter region 122 and the collector region 124 may be comprised of silicon with a P-type dopant while the base region 126 may be comprised of silicon with an N-type dopant.
The BJT 200 has larger current drive capability than the MOSFET 100 since the transconductance of a BJT is directly proportional to the current through the collector 124 of the BJT whereas the transconductance of a MOSFET is directly proportional to the square root of the current through the drain 104 of the MOSFET, as known to one of ordinary skill in the art of integrated circuits. However, unfortunately, the conventional BJT 200 occupies a relatively large area within an integrated circuit.
Because of the degradation of the current drive capability of a conventional MOSFET as the MOSFET is further scaled down and because of the large area occupied by a conventional BJT, another type of transistor is desired which has the desirable characteristics of both the higher current drive capability of the BJT and the smaller device area of the scaled down MOSFET.